The present invention relates to a nickel-base alloy product, nickel release from which is suppressed in low level even during a long period of use in high temperature water environments, and relates to a method of producing the same. This nickel-base alloy product is suited for use as structural members in nuclear reactors.
Nickel-base alloys, which have good mechanical properties, have been used as various members. In particular, nickel-base alloys superior in corrosion resistance are used as materials of nuclear reactor members which are exposed to high temperature water. Thus, Alloy 690 (60% Ni-30% Cr-10% Fe, trademark), for instance, is used in steam generators of Pressurized Water Reactors (PWRs).
These are to be used in nuclear reactor water environments, namely, high temperature water environments at about 300xc2x0 C., for at least several years to a period as long as several decades. Although nickel-base alloys are highly resistant to corrosion and the rate of corrosion thereof is slow, nickel is released from the alloys during a long period of use to form nickel ions, though in very small amounts.
In the process of circulation of reactor water, the released nickel is carried to the reactor core and irradiated with neutrons in the vicinity of the fuel. Upon irradiation with neutrons, nickel is converted to cobalt as a result of a nuclear reaction. Having a very long half-life, the cobalt continues to emit radiations for a long period of time. Therefore, if nickel is released in large amounts, workers engaged in periodic inspection, for instance, may be exposed to increased radiation doses.
To reduce the exposure dose is very important in using light water reactors for a long period of time. Therefore, in the art, measures have been taken to prevent the release of nickel from nickel-base alloys by improving the corrosion resistance on the material side or controlling the quality of nuclear reactor water.
JP Kokai S64-55366 discloses a method of improving the resistance to uniform corrosion of nickel-base alloy heating tubes. The method comprises annealing the tubes in a temperature range of 400-750xc2x0 C. in a high vacuum atmosphere of 10xe2x88x922 to 10xe2x88x924 torr in order to form an oxide film mainly composed of chromium oxides. JP Kokai H01-159362 discloses a method of improving the resistance to intergranular stress corrosion cracking by heat treatment in a temperature range of 400-750xc2x0 C. in an inert gas containing 10xe2x88x922 to 10xe2x88x924 volume % of oxygen to cause formation of an oxide film mainly composed of chromium oxide (Cr2O3).
JP Kokai H02-47249 and JP Kokai H02-80552 disclose methods of preventing the release of Ni and Co from stainless steel for heater tubes by heating the steel in an inert gas containing a specified amount of oxygen to cause formation of a chromium oxide film.
JP Kokai H03-153858 discloses a stainless steel resistant to the release in high-temperature water as a result of having, on the surface thereof, an oxide layer containing chromium-containing oxides in a higher proportion as compared with non-chromium-containing oxides.
The methods mentioned above all attempt to reduce the level of the release of metals by forming an oxide film mainly composed of Cr2O3 by heat treatment. However, the Cr2O3 film obtained by those methods loses their release preventing effect as a result of damage, for instance, during a long period of use. This is presumably due to an insufficient film thickness, an inadequate film structure, and low chromium content in the film.
It is an object of the present invention to provide a nickel-base alloy product showing a very low level of nickel release in high-temperature water environments over a long period of time, and a method of producing the same.
The gist of the present invention consists in a nickel-based alloy product as defined below under (1) and a method of producing the same as defined below under (2). In the following description, the percent value (%) expressing the content of each component means xe2x80x9c% by massxe2x80x9d, unless otherwise specified.
(1) A nickel-base alloy product having, on the surface thereof, an oxide film comprising at least two layers, namely a first layer mainly composed of Cr2O3 and having a chromium content of not less than 50% relative to the total amount of metal elements and a second layer occurring outside the first layer and mainly composed of MnCr2O4, wherein the grain size of Cr2O3 crystals in the first layer is 50 to 1,000 nm and the total oxide film thickness is 180 to 1,500 nm.
(2) A method of producing the nickel-base alloy product as specified above under (1) which comprises subjecting a nickel-base alloy product to oxide film formation treatment by maintaining the same at a temperature of 650 to 1,200xc2x0 C. in a hydrogen atmosphere or hydrogen-argon mixed atmosphere showing a dew point of xe2x88x9260xc2x0 C. to +20xc2x0 C. for 1 to 1,200 minutes.
It is desirable that the nickel-base alloy to serve as the base metal for producing the above product (1) is a nickel-base alloy containing C: 0.01-0.15%, Mn: 0.1-1.0%, Cr: 10-40%, Fe: 5-15% and Ti: 0.1-0.5%, with the balance being nickel and impurities.
In the above production method (2), the oxide film formation treatment mentioned above may be followed by further heat treatment by maintaining the product at 650-750xc2x0 C. for 300 to 1,200 minutes. Prior to oxide film formation treatment, the product may also be subjected to cold working. Cold working is effective in modifying the condition of the surface of the nickel-base alloy product in a manner such that chromium can diffuse more easily on the surface and in promoting the oxide film formation in the subsequent oxide film formation treatment.
In the present specification, the term xe2x80x9cnickel-base alloy productxe2x80x9d includes, within the meaning thereof, various products made of a nickel-base alloy, such as tubes or pipes, sheets or plates, rods or bars, and containers formed therefrom. The surface of a nickel-base alloy product means part or the whole of the surface of the product. When the product is a steam generator tube, for instance, the oxide film may be formed only on the inside surface of the product.
The grain size of Cr2O3 crystals in the first layer mainly composed of Cr2O3 is determined in the following manner. The nickel-base alloy product is dissolved in bromine-methanol solution, for instance, and three fields of the base metal side of the remaining oxide film is observed under Field Emission Electron Gun-Scanning Electron Microscope (FE-SEM) at a magnification of 20,000. The mean of the minor axis and major axis for each crystal is regarded as the grain size thereof. The average of such mean values is the crystal grain size.